1. Field of the Invention
The invention relates to a fall detection device that detects whether an apparatus is placed in a fall state, on the basis of acceleration, and to a magnetic disk drive and a portable electronic apparatus that each include the fall detection device.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2005-147899 (hereinafter, “the '899 application”) and Japanese Patent Publication No. 3441668 (hereinafter, “the '668 application”) describe apparatuses that detect whether an apparatus is placed in a fall state. FIG. 1 shows a configuration of a device according to the '899 application. This fall detection device includes an acceleration sensor 10, a differential circuit 42 that differentiates an acceleration detection signal outputted from the acceleration sensor 10 so as to output a differential signal, a first comparator 44 that determines whether the differential signal has reached a first predetermined threshold, a second comparator 46 that determines whether the differential signal has reached a second threshold higher than the first threshold, a voltage detector 50, and a processing circuit 48.
Suppose that the signal outputted from the acceleration sensor 10 has reached a predetermined setting value and is kept for a given time. In this case, when the differential signal reaches the first threshold, the fall detection device shown in FIG. 1 determines that the device is placed in a first processing state. When the differential signal reaches the second threshold, the fall detection device determines that the device is placed in a second processing state. Thus, for example, if a magnetic head of the hard disk drive is performing a recording operation when the fall detection device is placed in the first processing state, the fall detection device controls a safety operation such as suspending of the recording operation. If the fall detection device determines that the device is placed in the second processing state, it performs control, for example, so that the hard disk drive is put into a safer state.
A fall detection device according to the '668 application includes an acceleration sensor and a fall determination processing section and is configured to use both an acceleration and a differential value of the acceleration in order to determine whether the device is placed in a fall state.
However, the fall detection device according to the '899 application must use two thresholds simultaneously to determine a fall. This complicates a determination process. For this reason, particularly when a determination is made using software, problems occur, such as increasing the calculation load put on the CPU is increased and increasing the processing time.
Also, while the '899 application can determine a state in which the device is placed in a fall state and a state in which a shock has been given to the device, the '899 application has a problem that it is not possible to detect (predict a fall) a state in which the start of a fall is suspected. Therefore, it is not possible to perform a process of corresponding to the fall early.
Also, in the '899 application, an acceleration is used to make a determination. However, it is necessary to perform an offset adjustment on the acceleration sensor because an acceleration detection value obtained by the acceleration sensor includes an offset.
Also, for example, in a case where the device falls while it rotates, an acceleration caused by centrifugal force thereof is added. Therefore, the acceleration detection value is susceptible to the centrifugal force caused by the rotation. Thus, if the device falls while it rotates, a proper determination may not be made.
The fall detection device according to the '668 application also uses an acceleration and a differential value of the acceleration simultaneously. This complicates a determination process and causes the same problem as the problem with the '899 application. Also, a fall prediction cannot be made.
Also, variations in differential value of an acceleration is caused not only by the start of a fall but also by a simple, small shock (hereafter referred to as a “simple shock”) given when the device is handled. Therefore, it may be difficult to determine whether the device has actually started to fall or has been given a simple shock, on the basis of only variations in differential value of an acceleration.